Two twin size extra long flat sheets 66" x 102 " each, sewn together duvet cover style, will give you wiggle room to fold one mattress over the other when the cab over area is not needed as a bed.
For waterproofing when the children were young, we made sure the mattress was flipped to the waterproof side up. If your mattresses don't already have a waterproof side I would buy some cheap plastic painter's drop cloths and envelop each mattress separately.
Life is partly what we make it, and partly what it is made by the friends we choose.
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